The invention discloses an organic-inorganic compound sewage treatment agent and a preparation method thereof. The compound treatment agent is prepared from the following raw materials in parts by weight: 6 to 8 parts of gelatin, 9 to 14 parts of cysteine, 9 to 24 parts of thioglycolic acid, 20 to 24 parts of sodium lignosulfonate, 22 to 26 parts of carboxymethyl chitosan, 12 to 13 parts of urea,25 to 27 parts of limestone, 14 to 16 parts of sepiolite, 28 to 30 parts of bentonite, 25 to 30 parts of fly ash, 24 to 28 parts of quick lime, 13 to 15 parts of medical stone, 13 to 15 parts of aluminum sulfate, 15 to 17 parts of ferric chloride and 12 to 14 parts of polyethyleneimine xanthic acid. The compound treatment agent provided by the invention combines advantages of an inorganic mineralabsorbent and an organic biological high polymer flocculating agent; the removing rates on heavy metal, COD (Chemical Oxygen Demand), SS (Suspended Solids), TP (Total Phosphorus) and TN (Total Nitrogen) reach 95 percent or more respectively; the content of the heavy metal, the COD, the SS, the TP and the TN in treated river channel sewage meets type IV requirements in a table 1 of National Environmental Quality Standard for Surface Water of the People's Republic of China GB3838-2002.